Recently, in manufacturing of a semiconductor device, a non-plasma based dry etching referred to as a chemical oxide removal (COR) process is attracting attention as a method of performing a miniature etching which substitutes for a dry etching or a wet etching (see, e.g., Japanese Patent Application Publication Nos. 2005-39185 and 2008-160000).
In the COR process, HF gas, a gas mixture of HF and NH3, or the like is used in a case of etching a film made of a silicon-based material such as Si, SiO2, SiN or the like. In this case, the film is required to be etched with a good selectivity.
It is known that, among those materials, a film made of SiN can be etched with HF gas and a film made of SiO2 is difficult to be etched with HF gas but can be etched with the gas mixture of HF and NH3. Accordingly, in a case where the SiN film coexists with the SiO2 film, if the SiN film needs to be etched with a higher selectivity than the SiO2 film, HF gas is expected to be available.
However, when the SiN film is etched with HF gas, NH3 is produced as a reaction product and thus the SiO2 film is etched by the HF gas and the NH3 gas. Therefore, in practice, it is difficult to etch the SiN film with a higher selectivity than the SiO2 film by using HF gas. This problem is not limited to the case where the SiN film coexists with the SiO2 film and the SiN film is etched with HF gas. When a layer of a laminated film is etched with an etching gas, the above problem occurs if a different gas, contributing to an etching of a different layer of the laminated film, is produced.
In order to solve the above problem, a gas produced by a reaction may be purged by vacuum-exhausting the inside of a chamber and supplying a purge gas at an appropriate timing. However, purging the inside of the chamber takes time and mass productivity becomes poor.